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Tyrannosauroid Laurasian exchange in Lower Cretaceous

From: Ben Creisler

A new advance online paper:

Lindsay E. Zannoa & Peter J. Makovicky (2011)
On the earliest record of Cretaceous tyrannosauroids in western North
America: implications for an Early Cretaceous Laurasian interchange event.
Historical Biology (advance online publication)

The sudden appearance of Asian dinosaur clades within Lower Cretaceous
strata of western North America has long been recognised as a biotic
dispersion event related to initial establishment of a Beringian land
bridge. To date, uncertainty exists regarding the timing of the Early
Cretaceous Laurasian interchange event (EKLInE) and the pattern of
associated biotic dispersal. Here, we report a tyrannosauroid premaxillary
tooth (FMNH PR 2750) from the Cloverly Formation, Wyoming, USA, that pushes
back the earliest Cretaceous record of the clade in North America. Although
fragmentary, the tooth is consistent with mounting evidence for a pre-108
Ma initiation of EKLInE and earliest Albian emplacement of Beringia.
Previous authors have considered the Aptian/Albian of western North America
a depauperate dinosaur fauna, characterised by regional extinction and
diversity decline. Documentation of Albian tyrannosauroids in the region
indicates a more dynamic ecosystem than previously appreciated and marks an
early start to faunal mixing between immigrant and endemic dinosaur clades.
Finally, we find that the enamel microstructure of FMNH PR 2750 conforms to
the morphotype of tyrannosaurids, yet exhibits poor columnar
differentiation. This morphology bolsters prior interpretations on the
phylogenetic utility of enamel microstructure and suggests a trend of
increasing enamel complexity within Tyrannosauroidea.

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